The present invention relates to a heat-resisting high-strength Al-alloy that is excellent in heat-resistivity, hot-forgeability and stress-corrosion-cracking resistivity, and a method for manufacturing a structural member made of the same Al-alloy (for example, a piston for an internal combustion engine, a connecting rod, etc.) through a powder metallurgical process.
In an internal combustion engine for motor vehicles, in order to realize reduction of weight of a vehicle body, aluminium-alloy materials have been positively employed, and especially it is effective also for reducing an inertial force to form moving parts such as connecting rods, pistons or the like of aluminium-alloy materials. Such moving parts are required to have heat-resistivity and high strength because they are used under a severe condition at a high temperature, and in order to fulfil this requirement, there is a tendency of employing powder metallurgical products in which alloy elements can be added with a large freedom.
The inventor of this invention proposed previously jointly with two other co-inventors Al-alloy for powder metallurgical products in which high proportions of Si, Fe and other elements were added to Al aiming at improvements in a high-temperature strength, a Young's modulus, an abrasion-proofness and a heat-resistivity (See Japanese patent application No. 59-166979).
However, as a result of various subsequent investigations on the above-proposed Al-alloy containing Fe in the proportion range of 2.0.ltoreq.Fe.ltoreq.10 wt.%, it was seen that especially in the proportion range of Fe.gtoreq.6 wt.% it was necessary to make further improvements in hot-forging workability of a raw material for forging (in the form of a preshaped product), stress corrosion cracking resistivity of a finally shaped product, a density of a structural member and a strength of a structural member at 150.degree..about.200.degree. C.
More particularly, if the above-mentioned raw material for forging (Fe.gtoreq.6 wt.%) is subjected to high-speed hot-forging work (working speed=75 mm/sec or higher) that is equal to that in the case of duralumin, defects such as cracking or the like are liable to occur therein. Therefore, in order to improve the hot forging workability, various counter-measures in the forging process such as lowering of a working speed, raising of a metal mold temperature and the like have to be taken, hence a mess-productivity is degraded, and a manufacturing cost of parts would become high.
In addition, in the proportion range of Fe&lt;6 wt.%, although the structural member formed of the finally shaped product has a high strength as compared to that made of publicly known alloys (JIS AC8A, AC8B and AC8C: See Table-1) at a temperature in the proximity of 300.degree. C., at a temperature of 150.degree..about.200.degree. C. further improvements in a strength are desired.
TABLE 1 __________________________________________________________________________ (JIS H5202-1971: Al-alloys for metal-mold, sand-mold and shell castings) Chemical Composition (wt. %) Names of Symbols Cu Si Mg Zn Fe Mn Ni Ti Al Corresponding Alloys __________________________________________________________________________ AC8A 0.8.about.1.3 11.0.about.13.0 0.7.about.1.3 &lt;0.1 &lt;0.8 &lt;0.1 1.0.about.2.5 &lt;0.2 " AAA 332.0 Lo-ex AC8B 2.0.about.4.0 8.5.about.10.5 0.5.about.1.5 &lt;0.5 &lt;1.0 &lt;0.5 0.5.about.1.5 &lt;0.2 " Lo-ex AC8C 2.0.about.4.0 8.5.about.10.5 0.5.about.1.5 &lt;0.5 &lt;1.0 &lt;0.5 -- &lt;0.2 " AAF 332.0 __________________________________________________________________________
Furthermore, in the case where a connecting rod is formed of the above-proposed Al-alloy, there is a fear that stress corrosion cracking (according to the JIS stress corrosion cracking test) may arise at the locations where stress is continuously applied such as a pin-boss section (a smaller end portion) or a bearing-cap fastening section (a larger end portion) of a connecting rod, and this becomes a principal cause of lowering of durabilities of component parts in an engine in association with the trend of speed-up of an engine in the recent years.
Besides, since the above-proposed Al-alloy has a high density as compared to that of known alloys, the Al-alloy imposes a disadvantageous condition upon realization of light weight of a structural member.